I set up a sorority tank last week and everything has been going great. All the girls are happy and healthy. I exchanged one of the original ones I got the other day because she was constantly causing problem with the others. Well, after two days of what seemed fine, the one I replaced her with was dead this morning. The other fish appear fine, they all ate this morning, and everything. What should I do? I removed the dead one, but now I don't know if I should do a water change. Please help. Thank you.

If the fish you replaced her with was a newly acquired fish you should have had her quarantined for a couple weeks to make sure she was healthy. Also I recommend introducing a new fish to a sorority very slowly. Float the new fish for a long time not only to get acclimated to the water but to observe how she and the other fish interact to each other.
I don't think you need to panic about your other fish. If the fish you introduced was new it may have just been too much stress and shock to be put in with a bunch of females who were already established. Not sure if this is the scenario you had or not. This is what happened to me once.